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Characterization and formation mechanisms of fractures and their significance to hydrocarbon accumulation: A case study of Lower Ordovician mid-assemblage Formations in central Ordos Basin, China

鄂尔多斯盆地奥陶系中组合裂缝特征及其成藏意义

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Abstract

The lower Ordovician mid-assemblage Formations in the central Ordos Basin of China host prolific gas resources, and most hydrocarbon reserves are stored in naturally-fractured reservoirs. Thus, fracture pathway systems may have a significant impact on reservoir performance. This article focuses on the core- and laboratory-based characterization of fractures. Through the developmental degrees, extended scale, output state and filling characteristics of various types of fractures, the results show that there are three distinct fracture types: 1) nearly vertical fractures, 2) oblique fractures, and 3) horizontal fractures. Based on a systematic study of the characterization of reservoir space, the main geologic setting of natural gas accumulation and the regional tectonic background, type 1 is mainly driven by the tectonic formation mechanism, and type 3 and parts of low-angle fractures in type 2 are induced by the diagenetic formation mechanism. While recovered paleopressure for methane-rich aqueous inclusions trapped in fracture-filling cement indicates that the fracture opening and growth are consistent with gas maturation and charge and such high-angle fractures in type 2 are caused by the compound formation mechanism. The fractures to hydrocarbon accumulation may play a more significant role in improving the quality of reservoir porosity. Furthermore, connected fractures, dissolved pores and cavities together constitute the three-dimensional pore-cave-fracture network pathway systems, with faults serving as the dominant charge pathways of highly pressurized gas in the study area. Our results demonstrate that protracted growth of a pervasive fracture system is not only the consequence of various formation mechanisms but also intrinsic to quasi-continuous accumulation reservoirs.

摘要

近年来中国海相碳酸盐岩油气勘探发现了一批大型油气田, 其中鄂尔多斯盆地奥陶系马家沟组 中组合蕴藏着丰富的天然气资源, 储集空间类型主要有裂缝、裂缝-孔隙、孔隙和裂缝-孔洞型储层, 输导体系由断层、裂缝、溶蚀孔洞和晶间溶孔构成。然而有关奥陶系中组合裂缝特征、裂缝的形成机 制、古压力演化及其成藏意义的报道很少。基于钻井岩心观察、镜下微观分析、流体包裹体系统表征、 生烃增压数值模拟等研究结果, 结合天然气成藏地质背景, 厘定了鄂尔多斯盆地奥陶系中组合裂缝特 征、形成机制及其成藏意义。研究发现, 鄂尔多斯盆地奥陶系中组合碳酸盐岩裂缝主要可分为近垂直 缝、斜交缝和水平缝3 种类型。基于裂缝类型特征和发育的主控因素, 裂缝的成因机制归类为构造-成岩机制和复合成因机制。这些裂缝的形成期次主要有2 期: 第一期为中-晚侏罗世, 第二期为早白 垩世, 且在早白垩世时期大量发育的裂缝成为研究区奥陶系中组合天然气运聚成藏的优势通道和良好 储集空间。重点探讨了超压成藏演化所反映裂缝的形成和天然气运聚成藏过程, 明确了多期次构造运 动叠合和天然气超压-强超压运移充注构筑的裂缝网络体系与奥陶系中组合优势输导能力和良好储集 性能的关系, 建立了相应的机制与模式, 为下一步有利勘探靶区评价及开发部署提供理论指导和地质 依据。

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Acknowledgements

We are grateful to reviewers for the improvement of the submitted article. In particular, we acknowledge Changqing Oilfield Company of PetroChina for the help provided during the research work.

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Authors and Affiliations

  1. State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, 710069, China

    Zi-long Zhao  (赵子龙)

  2. Department of Geology, Northwest University, Xi’an, 710069, China

    Zi-long Zhao  (赵子龙)

  3. Shaanxi Key Laboratory of Petroleum Accumulation Geology, Xi’an Shiyou University, Xi’an, 710065, China

    Jing-zhou Zhao  (赵靖舟), Hai-jiao Ren  (任海姣), Jun Li  (李军) & Wei-tao Wu  (吴伟涛)

  4. School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an, 710065, China

    Jing-zhou Zhao  (赵靖舟), Hai-jiao Ren  (任海姣), Jun Li  (李军) & Wei-tao Wu  (吴伟涛)

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  1. Zi-long Zhao  (赵子龙)
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Correspondence to Jing-zhou Zhao  (赵靖舟).

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Foundation item: Project(2011ZX05007-004) supported by the National Sciences and Technologies, China; Project(41502132) supported by the National Natural Science Foundation of China

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Zhao, Zl., Zhao, Jz., Ren, Hj. et al. Characterization and formation mechanisms of fractures and their significance to hydrocarbon accumulation: A case study of Lower Ordovician mid-assemblage Formations in central Ordos Basin, China. J. Cent. South Univ. 25, 2766–2784 (2018). https://doi.org/10.1007/s11771-018-3952-z

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